<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title></title>
</head>
<style>
	.outline {
		margin: 50px;
		width: 200px;
		height: 200px;
		background: #FFCCCC;
		border-radius: 15px;
		outline: 5px solid #333;
		/*(√2 - 1) * r*/
		box-shadow: 0 0 0 5.8px #FFCCCC; 	
	}
	.no-outline {
		margin: 50px;
		width: 200px;
		height: 200px;
		background: #FFCCCC;
		border-radius: 15px;
		outline: 5px solid #333;
	}
</style>
<body>
	<div class="outline"></div>
	<br>
	<div class="no-outline"></div>
</body>
</html>